A、The temperature. B、The humidity. C、The air density. D、The air pressure. C

It is sometimes more difficult to get airplanes off the ground in extremely hot weather. But the problem is not the temperature itself or the humidity. It is the air density that can get in the way. Hotter air gets thinner. Thin air makes it more difficult for planes to lift and land safely. This is especially the case with smaller planes. Airplanes take off and stay in the air because of lift—the force from the movement of air under the plane’s wings that pushes it upward. Lou McNally is a professor of meteorology at Embry-Riddle Aeronautical University. When pilots try to take off in extreme heat the planes sometimes cannot lift before the runway ends. And, when pilots try to land when the air is too thin and hot, they can also run out of runway, McNally said.
To fly in high heat, planes must create more power and have larger wings. Airlines can take steps to be able to safely fly in extreme heat. One step is to reduce the plane’s weight by selling fewer seats. That is what American Airlines did during the recent Phoenix heat wave. Airlines can also reduce weight by lowering a plane’s fuel level.
Then the plane can stop somewhere cooler to refuel and take off again. Before they fly, pilots examine measures of what is called density altitude.
These include temperature and humidity. The density altitude is the main issue, but there are other risks that come with flying in hot temperatures.
Questions 16 to 18 are based on the passage you have just heard.
16. What makes planes difficult to get off the ground in extremely hot weather?
17. What can airlines do to safely fly in extreme heat?
18. What does the speaker mainly talk about?